There has been an ever increasing need in the automotive industry to reduce the weight of automobiles and hence reduce gasoline consumption. This need has resulted in the replacement of many metal parts with plastic parts. See, e.g., Welch, U.S. Pat. No. 3,850,474. While many of the plastic parts presently used in automobiles have been produced by well known injection molding and extrusion techniques, it would be very desirable if these plastic parts could be produced by the same technique, and even the same machinery used for making the metal parts--i.e., stamping. However, for a variety of reasons, any significant conversion from stamping metal parts to stamping plastic parts has not occured.
As discussed in Canadian Pat. No. 1,013,527, the following sequence of steps are typically employed in stamping:
(1) the plastic sheet or blank is heated to a temperature which is above the melting temperature of softening point and below the decomposition point of the plastic;
(2) the heated sheet is transferred to a cold mold of a mechanical stamping press where a set of dies therein has the desired configuration of the intended shaped articles;
(3) the press is closed for a period of time sufficient to cause the sheet to fill the die cavity and to cool the part to retain the desired shape of the die cavity; and
(4) the press is opened and the molded article is ejected.
An alternative, related form of fabrication, used in so-called "thermoforming" of plastic sheets is "vacuum forming" wherein the softened plastic sheet is drawn into a mold by air pressure and a mating portion of the mold is not required.
One of the major problems in stamping thermoplastic sheets is controlling the temperature during transfer to the dies and during closure of the dies. The desired thermoplastic materials such as polypropylene have a relatively sharp cyrstalline melting point. Accordingly, when temperatures exceed about 5.degree. above the melting point of the polymer, the sheet sags and becomes difficult if not possible to transfer from the heater to the mold. However, if the temperature is too low, e.g., only 1 or 2 degrees above the melting point, then the polymer will not flow into the interstices of the die and will snap back when the die is removed. This very narrow control range of 2 to 5 degrees above the crystalline melting point, while frequently manageable in a controlled laboratory environment, cannot be readily adapted to commercial practices. What is needed is a means to permit heating the polymer to higher temperatures above the melting temperature or softening point that is commercially manageable without also resulting in a drastic loss of melt strength and problems with sheet transfer and sagging sheets. Generally, what is required is a broader "temperature window" for processing; namely, a larger range of temperatures within which a satisfactory balance between sheet sag and in-mold flow is maintained. Further, there is also a need for a polymer sheet which can be more easily thermoformed into an article having an intricate pattern.